In the electrical power transmission and telecommunications industries, it is necessary to install power transmission cables and communications cables such as fiber optic cables over long distances. Such cables may be either suspended in the air between tower structures or buried underground. Tower structures, however, are expensive and unsightly and, furthermore, cables suspended from them are vulnerable to weather and flying objects such as low flying aircraft. Moreover, fiber optic cable is relatively non-resilient and can be easily broken from wind currents if suspended in the air. Therefore, fiber optic cable is not typically suspended in the air.
As an alternative to suspending cable in the air, power transmission cables and fiber optic cables are often buried underground. Buried cable typically requires that a trench be dug, the cable be laid in the trench, and soil be replaced in the trench. Such underground burying of cable can be time-consuming and, consequently, expensive, even more expensive than suspending the cable. To reduce the cost of burying cable underground, techniques have been developed to run cable through existing buried pipes, such as pipelines.
One technique that has been used to run cable in pipelines is to attach a pulling eye or pulling grip to one end of the cable. A pulling cable or rope is then threaded through the pipe or conduit and affixed to the pulling eye or pulling grip. A cable puller or winch is then used to pull the pulling cable or rope, followed by the transmission cable, through the pipeline or conduit. Prior to installation, to reduce friction and tension, the transmission cable is normally coated with a lubricant such as soap, bentomite clay or talc mixtures with water.
An alternative technique utilizes compressed air to propel the line or filament through the pipeline or conduit. A plug, which is configured to create an air-tight seal when inserted into the pipeline or conduit, is affixed to the end of the line or filament. The plug is inserted into the pipeline or conduit and compressed air is applied which propels the plug and the attached line or filament.
A major disadvantage with utilizing the pulling process or the compressed air process is that stress placed upon the line during installation limits the length of line that can be installed at any one time. This is true particularly for large diameter lines. As a result, the installation of relatively long lines in a pipeline or conduit requires the installation of several individual line segments which are spliced together. The installation of individual segments and the required splicing process is very expensive and time consuming.
It has been disclosed in U.S. Pat. No. 5,011,332 entitled "Apparatus and Method for Introducing a Cable-Like Element into a Pipe and Elements Suitable Therefore" issued Apr. 30, 1991 to Kunze et al. that a jacket may be positioned around the line to be installed in a pipeline. The jacket is provided with a profile to induce turbulence in a pipe to cause the line to oscillate in the pipe. The line is also matched with the fluid in the pipeline to be buoyant in the fluid so that contact of the jacket with the inner walls of the pipe is minimized.
There are several disadvantages associated with such a use of such a jacketed-line approach. For example, large diameter cable-like elements (e.g., two to six inches in diameter) can be very heavy, commonly from 10-20 pounds per linear foot. The installation of such heavy lines requires a large diameter jacket which would displace a significant percentage of the total cross-sectional fluid flow area of the pipeline. The jacket would reduce the total number of lines that could be installed, or reduce the volume of fluid thru-put, if the pipeline was to be utilized simultaneously for fluid transmission. Moreover, the jacket cannot be removed; therefore, such a line continues indefinitely to occupy a large portion of the pipeline cross-section and to oscillate in the pipeline resulting in increased turbulence and friction loss, and hence pressure loss, in the flow of fluid in the pipeline.
Accordingly, a continuing search has been directed to the development of improved methods which can be used to install cable, particularly heavy large-diameter cables, through pipes without requiring the cable to be installed in small segments that must be spliced together, and while minimizing the pressure loss of fluid flowing through the pipeline.